LED screen devices are widely used in large screen display for business, architectures, and stages. In order for easy mounting and transporting, current LED devices for large screen display are mostly assembled by several smaller LED screens. U.S. Pat. No. 6,677,918 published such a LED display system. The large screen includes several rigid frames assembled together On every rigid frame, a plurality of rigid supporting stripes are disposed perpendicularly with equal intervals. And a plurality of RGB pixel units are disposed with equal intervals on every rigid supporting stripe, so as to form a RGB pixel matrix. The display system also includes signal transmission lines, and power lines connected to respective pixel units, and a control system. The control system receives outside video signals, converts the signals into digital signals, and transfers the correct pixel display data to the LED driver after the process of the computer, so as to control the operation of every pixel units. Because every RGB pixel unit consists of a red, a blue, and a green LED, it is able to generate any visible color including white and black. Hence, the large screen can display all kinds of still or moving color pictures. The invention used the method of assembling for large screens, however, all frames are rigid, and of the same size, so it is very inconvenient for mounting and transporting and the control system is relatively complicated. U.S. Pat. No. 6,362,801 published another LED display system, whose large screen is of flexible nets. The pixel units are disposed with equal intervals on the nets to form regular pixel matrix. However, the wirings are too complicated, hence hard for maintenance.